This invention relates to catalysts useful for cracking heavy hydrocarbon feed stocks, such as residuum (resid) and residuum-containing feeds, that contain high levels of nickel and vanadium contaminants. In particular, the invention is directed to improvements in zeolitic fluid cracking catalysts (FCC) produced by an in-situ reaction wherein preformed microspheres obtained by calcining microspheres composed of a mixture of hydrated kaolin, a dispersible boehmite alumina, binder, and kaolin or other clay calcined through its characteristic exotherm, undergo chemical reaction with sodium silicate solution to form crystals of zeolite and a porous silica/alumina matrix. The catalyst is metals tolerant, has good catalytic selectivity and is especially useful as a catalyst for cracking resids and resid-containing feeds.
Since the 1960""s, most commercial fluid catalytic cracking catalysts have contained zeolites as an active component. Such catalysts have taken the form of small particles, called microspheres, containing both an active zeolite component and a non-zeolite component in the form of a high alumina, silica-alumina matrix. The non-zeolitic component or matrix is known to perform a number of important functions, relating to both the catalytic and physical properties of the catalyst. Oblad described those functions as follows:
xe2x80x9cThe matrix is said to act as a sink for sodium in the sieve thus adding stability to the zeolite particles in the matrix catalyst. The matrix serves the additional function of: diluting the zeolite; stabilizing it towards heat and steam and mechanical attrition; providing high porosity so that the zeolite can be used to its maximum capacity and regeneration can be made easy; and finally it provides the bulk properties that are important for heat transfer during regeneration and cracking and heat storage in large-scale catalytic cracking.xe2x80x9d A. G. Oblad Molecular Sieve Cracking Catalysts, The Oil And Gas Journal, 70, 84 (Mar. 27, 1972).
In prior art fluid catalytic cracking catalysts, the active zeolitic component is incorporated into the microspheres of the catalyst by one of two general techniques. In one technique, the zeolitic component is crystallized and then incorporated into microspheres in a separate step. In the second technique, the in-situ technique, microspheres are first formed and the zeolitic component is then crystallized in the microspheres themselves to provide microspheres containing both zeolitic and non-zeolitic components.
For many years a significant proportion of commercial FCC catalysts used throughout the world have been made by in-situ synthesis from precursor microspheres containing kaolin that had been calcined at different severities prior to formation into microspheres by spray drying.
U.S. Pat. No. 4,493,902, the teachings of which are incorporated herein by cross-reference, discloses novel fluid cracking catalysts comprising attrition-resistant, high zeolitic content, catalytically active microspheres containing more than about 40%, preferably 50-70% by weight Y faujasite and methods for making such catalysts by crystallizing more than about 40% sodium Y zeolite in porous microspheres composed of a mixture of two different forms of chemically reactive calcined clay, namely, metakaolin (kaolin calcined to undergo a strong endothermic reaction associated with dehydroxylation) and kaolin clay calcined under conditions more severe than those used to convert kaolin to metakaolin, i.e., kaolin clay calcined to undergo the characteristic kaolin exothermic reaction, sometimes referred to as the spinel form of calcined kaolin. In a preferred embodiment, the microspheres containing the two forms of calcined kaolin clay are immersed in an alkaline sodium silicate solution, which is heated, preferably until the maximum obtainable amount of Y faujasite is crystallized in the microspheres.
In practice of the ""902 technology, the porous microspheres in which the zeolite is crystallized are preferably prepared by forming an, aqueous slurry of powdered raw (hydrated) kaolin clay (Al2O3:2SiO2:2H2O) and powdered calcined kaolin clay that has undergone the exotherm together with a small amount of sodium silicate which acts as fluidizing agent for the slurry that is charged to a spray dryer to form microspheres and then functions to provide physical integrity to the components of the spray dried microspheres. The spray dried microspheres containing a mixture of hydrated kaolin clay and kaolin calcined to undergo the exotherm are then calcined under controlled conditions, less severe than those required to cause kaolin to undergo the exotherm, in order to dehydrate the hydrated kaolin clay portion of the microspheres and to effect its conversion into metakaolin, this results in microspheres containing the desired mixture of metakaolin, kaolin calcined-to undergo the exotherm and sodium silicate binder. In illustrative examples of the ""902 patent, about equal weights of hydrated clay and spinel are present in the spray dryer feed and the resulting calcined microspheres contain somewhat more clay that has undergone the exotherm than metakaolin. The ""902 patent teaches that the calcined microspheres comprise about 30-60% by weight metakaolin and about 40-70% by weight kaolin characterized through its characteristic exotherm. A less preferred method described in the patent, involves spray drying a slurry containing a mixture of kaolin clay previously calcined to metakaolin condition and kaolin calcined to undergo the exotherm but without including any hydrated kaolin in the slurry, thus providing microspheres containing both metakaolin and kaolin calcined to undergo the exotherm directly, without calcining to convert hydrated kaolin to metakaolin.
In carrying out the invention described in the ""902 patent, the microspheres composed of kaolin calcined to undergo the exotherm and metakaolin are reacted with a caustic enriched sodium silicate solution in the presence of a crystallization initiator (seeds) to convert silica and alumina in the microspheres into synthetic sodium faujasite (zeolite Y). The microspheres are separated from the sodium silicate mother liquor, ion-exchanged with rare earth, ammonium ions or both to form rare earth or various known stabilized forms of catalysts. The technology of the ""902 patent provides means for achieving a desirable and unique combination of high zeolite content associated with high activity, good selectivity and thermal stability, as well as attrition-resistance.
The aforementioned technology has met widespread commercial success. Because of the availability of high zeolite content microspheres which are also attrition-resistant, custom designed catalysts are now available to oil refineries with specific performance goals, such as improved activity and/or selectivity without incurring costly mechanical redesigns. A significant portion of the FCC catalysts presently supplied to domestic and foreign oil refiners is based on this technology.
U.S. Pat. Nos. 5,023,220 and 5,395,809, assigned to the present assignee are further examples of patents which teach the formation of catalytic FCC microspheres from mixtures of hydrous kaolin, metakaolin and kaolin that has been calcined through the characteristic exotherm. These mentioned patents are herein incorporated by reference in their entirety.
Improvements in cracking activity and gasoline selectivity of cracking catalysts do not necessarily go hand in hand. Thus, a cracking catalyst can have outstandingly high cracking activity, but if the activity results in a high level of conversion to coke and/or gas at the expense of gasoline the catalyst will have limited utility. Catalytic cracking activity in present day FCC catalysts is attributable to both the zeolite and non-zeolite (e.g., matrix) components. Zeolite cracking tends to be gasoline selective. Matrix cracking tends to be less gasoline selective. After appropriate ion-exchange treatments with rare earth cations, high zeolite content microspheres produced by the in situ procedure described in the ""902 patent are both highly active and highly gasoline selective. As zeolite content of these unblended microspheres is increased, both activity and selectivity tend to increase. This may be explained by the decrease in matrix content with increase in zeolite content and the decreasingly prominent role of nonselective matrix cracking.
In recent years the oil refining industry has shifted to processing a larger quantity of resid due to the changing product slate and price structure of crude oil. Since the early 1980""s many refiners have been processing at least a portion of resid as a feedback in their units and several now run a full resid cracking program. Processing resid can drastically alter yields of valuable products in a negative direction relative to a light feed.
Several factors are important to resid catalyst design. It is highly favorable if the catalyst can upgrade bottoms, minimize coke and gas formation, maximize catalyst stability, and minimize deleterious contaminant selectivity due to metal contaminants in resid feedstocks such as nickel and vanadium. While in-situ catalysts are commercially valuable, none of these in-situ catalysts possessed such a combination of properties when used to crack resid feedstocks.
Following the inception of catalytic cracking by Houdry in the early 1900""s where an acid treated clay was used, the first revolution in the art of catalyst technology was the use of synthetic silica-alumina. The use of silica-alumina which had much more acidic Bronsted and Lewis acid sites increased the cracking activity and selectivity of the process over the clays. The second revolution came with the advent of zeolites and the discovery that they could be applied to cracking. The clear advantage of the zeolite was that the non-selective cracking to coke and gas was greatly reduced owing to the discrete pore structure of the crystalline zeolite and the shape selective chemistry which they provided. With the thrust in modern refining to limit the amount of coke and gas so as to maximize gasoline production the designed use of silica-alumina in cracking catalysts has decreased (see A. A. Avidan in: Fluid Catalytic Cracking: Science and Technology. Studies in Surface Science and Catalysis, Vol. 76. Magee, J. S. and Mitchell, M. M. Eds.; Elsevier, Amsterdam; 1993). The use of added aluminas has also found merit in helping to boost a catalyst""s activity since pure aluminas also posses acidic sites. The relative activity of a catalyst is roughly proportional to the total quantity of acid sites present. Unfortunately alumina characteristically contains a large fraction of Lewis acid sites relative to Bronsted type sites. Lewis sites have been shown to be largely involved in the chemistry of hydride abstraction and coke formation (see Mizuno, et al. in Bulletin of the Chemical Society of Japan, Vol. 49, 1976, pp. 1788-1793).
Fluid cracking catalysts which contain silica-alumina or alumina matrices are termed catalysts with xe2x80x9cactive matrixxe2x80x9d. Catalysts of this type can be compared with those containing untreated clay or a large quantity of silica and which are termed xe2x80x9cinactive matrixxe2x80x9d catalysts. Work done by Otterstedt, et al. (Applied Catalysts, Vol. 38, 1988, pp. 143-155.) clearly shows the disadvantage of active matrices for coke and gas production sometimes producing twice as much as the inactive formulation.
Aluminas have long been used in hydrotreating and reforming catalyst technology (see P. Grange in Catalysis Reviewsxe2x80x94Science and Engineering, Vol. 21, 1980, p. 135). Aluminas, and particularly transition aluminas, in addition to displaying acidic character also posses high surface areas typically on the order of several hundred meters squared per gram. They may be well suited for catalyst applications such as those mentioned where a metallic component is to be supported on the substrate surface (alumina in this case). The high surface area of the host material above allows for a more uniform, dispersed arrangement of the metal. This leads to smaller metal crystallites and helps to minimize metal agglomeration. Metal agglomeration or sintering is a leading cause of loss of activity since the activity for metal catalyzed reaction is proportional to the exposed metal surface area. When the metal sinters metallic surface area is lost and so is activity. In relation to catalytic cracking, despite the apparent disadvantage in selectivity, the inclusion of aluminas or silica-alumina has been beneficial in certain circumstances. For instance when processing a hydrotreated/demetallated vaccum gas oil (hydrotreated VGO) the penalty in non-selective cracking is offset by the benefit of cracking or xe2x80x9cupgradingxe2x80x9d the larger feed molecules which are initially too large to fit within the rigorous confines of the zeolite pores. Once xe2x80x9cprecrackedxe2x80x9d on the alumina or silica-alumina surface, the smaller molecules may then be selectively cracked further to gasoline material over the zeolite portion of the catalyst. While one would expect that this precracking scenario might be advantageous for resid feeds they are unfortunately characterized for the most part as being heavily contaminated with metals such as nickel and vanadium and to a lesser extent, iron. When a metal such as nickel deposits on a high surface area alumina such as those found in typical FCC catalysts, it is dispersed and participates as highly active centers for the catalytic reactions which result in the formation of contaminant coke (contaminant coke refers to the coke produced discretely from reactions catalyzed by contaminant metals). This additional cokes exceeds that which is acceptable by refiners.
Loss of activity or selectivity of the catalyst may also occur if the metal contaminants such as nickel, vanadium, from the hydrocarbon feedstock, deposit onto the catalyst. These metal contaminants are not removed by standard regeneration (burning) and contribute markedly to undesirably high levels of hydrogen, dry gas and coke and reduce significantly the amount of gasoline that can be made. Contaminant metal levels are particularly high in certain feedstocks, especially the more abundant heavier crudes. As oil supplies dwindle, successful economic refining of these heavier crudes becomes more urgent. In addition to reduced amounts of gasoline, these contaminant metals contribute to much shorter life cycles for the catalyst and an unbearably high load on the vapor recovery system. Deposited nickel and vanadium species have an intrinsic dehydrogenation activity which leads to the formation of coke and gas, two undesirable products. Furthermore, vanadium assists in destroying the crystallinity of the sieve. This leads to a loss of catalytic activity and to the formation of certain silica-alumina species which tend to promote the formation of coke and gas. The increased expense of refining metal-contaminated feedstocks due to the aforementioned factors lays a heavy economic burden on the refiner. Therefore, much effort has been spent in finding means to modify the catalyst or feedstock in such a way as to passivate the aforementioned undesirable effects of the metal contaminants.
Commonly assigned U.S. Pat. No. 5,559,067 addresses the problem of providing a resid FCC catalyst made by the in-situ route which can upgrade bottoms, minimize coke and gas formation, maximize catalyst stability and minimize deleterious contaminant selectivity due to contaminant metals. The resid FCC catalyst of the patent achieves metal tolerance in a manner considered to be relatively inexpensive to practice and does not result in the use of environmentally toxic additives such as the use of prior art technologies for achieving metals tolerance such as those involving the use of antimony. In accordance with the patent, microspheres comprising hydrous kaolin clay, gibbsite (alumina trihydrate), spinel, and a silica sol binder are prepared, the microspheres calcined to convert the hydrous kaolin component to metakaolin and the calcined microspheres reacted with an alkaline sodium solution into crystallized zeolite Y and ion exchanged.
During the conversion of hydrous kaolin to metakaolin, gibbsite also undergoes transformation to a transition alumina. Transition alumina may be defined as any alumina which is intermediate between the thermodynamically stable phases of gibbsite, bayerite, boehmite, and nordstandite on one end of the spectrum and alpha alumina or corrundum on the other. Such transition aluminas may be viewed as metastable phases. A scheme of the transformation sequence can be found in the text: Oxides and Hydroxides of Aluminum by K. Wefers and C. Misra; Alcoa Technical Paper No. 19, revised; copyright Aluminum Company of America Laboratories, 1987.
A novel, in-situ fluid cracking catalyst is provided which is useful in cracking feeds that contain nickel and vanadiaum. The FCC catalyst of this invention is made from microspheres which initially contain kaolin, binder, and a dispersible boehmite alumina. The microsphere is subsequently converted using standard in-situ Y zeolite growing procedures to make a Y-containing catalyst. Exchanges with ammonium and rare earth cations with appropriate calcinations provides an FCC catalyst that contains a transitional alumina obtained from the boehmite.
Preparation of the novel fluid cracking catalyst, in accordance with an aspect of this invention, involves an initial step of preparing microspheres comprising hydrous kaolin clay and/or metakaolin, a dispersible boehmite (Al2O3, H2O), optionally spinel and/or mullite, and a sodium silicate or silica sol binder. The microspheres are calcined to convert any hydrous kaolin component to metakaolin. The calcination process transforms the dispersible boehmite into a transitional alumina phase. The calcined microspheres are reacted with an alkaline sodium silicate solution to crystallize zeolite Y and ion-exchanged. The transitional alumina phase that results from the dispersible boehmite during the preparative procedure and which forms the matrix of the final catalyst, passivates the Ni and V that are deposited on to the catalyst during the cracking process, especially during cracking of heavy residuum feeds. This results in a substantial reduction in contaminant coke and hydrogen yields. Contaminant coke and hydrogen arise due to the presence of Ni and V and reduction of these byproducts significantly improves FCC operation.